Satellites and spacecraft frequently comprise liquid propellant thrusters for their high intensity thrust requirements. The liquid propellants intended to feed these thrusters are stored in tanks, the design of which has to take into account environmental conditions in which these craft are required to travel and notably an almost zero gravity, or a gravity substantially lower than the Earth's gravity variably oriented.
In such a tank, a propellant in liquid phase, contained in the tank, has to be able to be dispelled through an exhaust orifice. Generally, the tank contains both a propellant in liquid phase and gases (propellant vapors, inert pressurizing gas). The liquid expulsion device has to favor the expulsion of liquid from the tank through the evacuation orifice, while preventing the output through this orifice of the gases present in the tank, which could be detrimental to the correct operation of the propulsion system (risk of explosion).
Now, the absence of a constant direction of gravity enabling the liquid to collect at a low point prevents the use of traditional devices for liquid recovery and separation from gas bubbles.
In other, so-called “cold gas”, satellite propulsion systems, a fluid is stored in a tank in diphasic state. The tank is heated to generate gas, which is discharged through piping and released at nozzles, thus creating a thrust on the satellite. Unlike the preceding systems in which liquid had to be expelled without bubbles, here it is necessary to discharge gas without droplets. The issue is then how to retain the liquid in the tank, by using a liquid retention device.
Various liquid expulsion or retention devices have therefore been designed to remedy this problem. Prior art in this field that can be cited includes the European patent EP 0 132 175 B2, published on Jan. 23, 1985, from the company Matra, which deals with a liquid storage tank with capillary containment.
Another patent that can be cited is FR 2 655 956 from the SociétéEuropéenne de Propulsion, published on 21 Jun. 1991 which targets a tank with shell capillary effect.
Finally, the patent EP 1 868 891 from Astrium SAS published on 26 Dec. 2007 describes the use of a liquid retention device used in a cold gas propulsion system.
According to the prior art, the core of the liquid expulsion or retention devices generally consists of a capillary structure, called sponge, whose function is to retain the liquid by capillarity when operating in conditions of weightlessness when the satellite is placed in its orbit. This sponge is placed close to the liquid propellant discharge orifice and communicates with this orifice.
The sponge of the liquid expulsion/retention device has a volume that is sufficient to retain a quantity of liquid propellant that is at least equal to, and generally greater than, a predetermined quantity. In the case of an expulsion device this predetermined quantity is the quantity needed to carry out a type of maneuver during which the propulsion system has to operate continuously for a predefined duration, even though the acceleration applied to the tank during said maneuver is applied in a direction such that the liquid propellants are not necessarily pressed toward the bottom of the tank in which the liquid discharge orifice is situated. Between two such maneuvers, the craft is once again at zero gravity, and the sponge is recharged with liquid while discharging the gas which is located therein.
In a prior art arrangement, illustrated by FIG. 1, the sponge of a liquid expulsion/retention device is made up of a large number of blades 11, for example metallic, arranged to offer a capillary gradient in a direction favoring the expulsion of the liquid through a liquid extraction orifice 12, provided for this purpose.
A quantity representative of the capillarity at a point of the sponge is the mean curvature of the free surface between liquid and gas at this point. Capillary gradient in a given direction should be understood to mean the spatial derivative of the capillarity in that direction. A positive gradient in a given direction means that the capillarity increases in that direction. When the gravitational or acceleration forces are weak, the liquid contained in this sponge tends naturally to move toward the area of strongest capillarity, in the direction in which the capillary gradient is positive, unlike the gas which tends naturally to move toward the zone of weakest capillarity, in the direction in which the capillary gradient is negative. The liquid extraction orifice 12 is arranged in immediate proximity to the zone of maximum capillarity of the sponge, called “expulsion/retention zone”. The zone of the device through which the gas can escape is a so-called peripheral zone, generally far from the liquid expulsion/retention zone.
The blades 11, which can be planar or corrugated, can be arranged radially in such a way as to converge toward the center of the tank 10, thus creating a capillary gradient in that direction. The liquid is then collected in the central part of the tank and routed by capillarity toward the liquid discharge orifice.
In another example, illustrated by FIG. 2 the blades 11 are arranged fanwise within a housing 20. They are arranged substantially in the direction of the axis of the tank (called vertical direction and denoted Z) in a way such that they come closer together as they approach the liquid extraction orifice 12.
The limitations of the sponges of blade-based liquid expulsion/retention devices, as illustrated in FIGS. 1 and 2 in particular, and of other devices of this type according to the prior art, are manifold.                Obtaining a good capillary pressure requires the blades to be sufficiently close together (typically 1 to 5 mm) which requires a large number of blades (typically a hundred or so).        The geometrical tolerances imposed on fitting of the blades are restrictive (typically less than 0.5 mm).        Assembling this large number of parts according to the prior art (for example by welding) with restrictive tolerances is difficult to perform and costly.        The type of parts used to produce the sponge according to the prior art limits the design possibilities for this kind of device.        